1. Field of the Invention
The present invention relates to an auto focus system having an AF frame auto-tracking function, and more particularly, to an auto focus system having an AF frame auto-tracking function that allows an AF area (AF frame), which is the target range of an object brought into focus by auto focus (AF), to follow a predetermined object moved on an imaging screen.
2. Description of the Related Art
Broadcasting or commercial television camera systems having an auto focus function for auto focusing on a desired object have been known. This type of camera system generally uses contrast-type AF. In the contrast-type AF, the camera system detects the level of the contrast of the captured image, and controls the focus of an imaging optical system such that the contrast becomes the maximum (the largest).
In general, the target range of an object that is brought into focus by AF is not the entire screen of the captured image, but is limited to a portion of the screen, which is called an AF area. Focus control is performed such that the contrast of the captured image (object image) in the AF area becomes the maximum, thereby focusing on the object in the AF area.
In the specification, a frame indicating the outline of the AF area is referred to as an AF frame.
JP-A-2006-258944 (corresponding to US 2006/210260 A) discloses an auto focus system having an AF frame auto-tracking function that allows the AF frame to automatically follow a desired object moved on the screen of the captured image such that the object is continuously in focus. The auto-tracking of the AF frame is performed by detecting the image of an object to be tracked from the captured image and moving the AF frame to the detected position. As a method of detecting the image of an object to be tracked, JP-A-2006-258944 discloses a method of storing the image of the object to be tracked as a reference pattern and detecting an image matched with the reference pattern from the captured image using a pattern matching process.
However, in some cases, the user wants to use the AF frame auto-tracking function to track, for example, a person's face. In this case, when the size of the AF frame is excessively larger than that of the face (face image) in the captured image, a large portion of the background image is included in the AF frame. When a large number of high frequency components are included in the background image, the imaging device focuses on the background in the contrast-type AF. In addition, the image of a reference pattern is set to an initial image in the AF frame or an image in a range corresponding to the AF frame in the pattern matching process. Therefore, when the background image of the face image varies, it is difficult to detect an image matched with the image of the reference pattern in the pattern matching process. Therefore, it is difficult to move the AF frame to track the face image.
Even though the AF frame auto-tracking function starts to set the AF frame to an appropriate size corresponding to the size of the face image to be tracked, the size of the face image in the AF frame is changed when a zoom operation of an imaging lens (imaging optical system) is performed or when the distance between a camera and an object varies. Therefore, for example, when the zoom operation is performed to a wide side, the size of the face image in the captured image is reduced, which causes the above-mentioned problems. When the zoom operation is performed to a telephoto side, a portion of the face image is within the range of the AF frame, and high frequency components are reduced. As a result, it is difficult to perform focusing on the object by AF. In addition, in the pattern matching process, it is difficult to detect an image matched with the image of the reference pattern. Therefore, it is difficult to move the AF frame to track the face image.
Further, it takes time and effort for the user to set and change the AF frame according to the size of the face image to be tracked. It is difficult for the user to set and change the AF frame.